This invention relates to a crystal oscillator, for example, a crystal oscillator for use in a mobile communications apparatus.
A crystal oscillator of this type is an essential part for generating an oscillator signal for control of signal reception and transmission between mobile communications apparatus or the like. Such a crystal oscillator is required to have a very small volume as the mobile communications apparatus is constructed smaller and to have a reference clock signal having a stable frequency even if being used under an environment where ambient temperature drastically changes.
In response to the request for stabilized frequency, temperature compensation is performed to make the oscillating frequency constant and independent of ambient temperature against the intrinsic temperature-frequency variation characteristic of the crystal oscillator (e.g., an AT-cut crystal oscillating element has a temperature-frequency variation characteristic represented by a three-dimensional curve). In order to conduct this temperature compensation in a small crystal oscillator at a high accuracy, variations in oscillating frequency of a crystal oscillating element which varies with ambient temperature is flattened as a whole by use of an oscillating inverter and an IC (Integrated Circuit) chip capable of sensing a temperature, storing temperature compensation data for specified temperatures, converting the voltage, performing varicap diode function, and performing control. Specifically, oscillating frequency variations are flattened by setting the capacity value of varicap diode performance at a suitable predetermined value based on temperature compensation data.
In recent years, a crystal oscillator provided with a control circuit for compensating a temperature-frequency variation characteristic is welded to a printed circuit board by reflowed solder together with other electronic devices and devices. Accordingly, there have been proposed crystal oscillators formed with external terminal electrodes on a surface of a main body thereof. The conventional crystal oscillator provided with external terminal electrodes is formed with a cavity for accommodating a crystal oscillating element and a control circuit. Specifically, the control circuit including an IC chip is arranged in a lower portion of the cavity and the crystal oscillating element is arranged in an upper portion of the cavity or atop the control circuit. In other words, the both members are placed in the same space. Finally, the cavity is hermetically sealed by a metal cover. However, since the frequency variation characteristic of the crystal oscillating element varies due to deposition of impurities on the surface thereof or other causes, it is desirable to hermetically seal the crystal oscillating element in a space provided specially therefor. Further, it is desired to arrange the IC chip in a location which is not subject to the unnecessary heat.
Japanese Unexamined Patent Publication No. 10-28024 discloses a small crystal oscillator provided with external terminal electrodes on a surface thereof which satisfies the aforementioned demands and is able to highly accurately conduct a temperature compensation. This crystal oscillator includes a plate-like substrate and a rectangular hollow member attached on a bottom surface of the plate-like substrate. The rectangular hollow member has a rectangular space. The plate-like substrate and the rectangular hollow member defines a cavity. A crystal oscillating element is mounted on a top surface of the plate-like substrate while a control circuit is provided on a bottom surface of the plate-like substrate and in the cavity.
More specifically, as shown in FIGS. 11 to 13, the crystal oscillator is mainly comprised of a main body 51, a rectangular crystal oscillating element 52, an IC chip 53 constituting a control circuit and a metal cover 54. In this crystal oscillator is used the main body 51 which is an integral assembly of a plate-like ceramic substrate 55 and a rectangular hollow member 56 attached on the bottom surface of the substrate 55. The rectangular hollow member 56 has a rectangular space. Accordingly, a cavity 57 is defined in a lower portion of the main body 51.
In the ceramic substrate 55 partitioning the top surface of the main body 51 and the ceiling surface of the cavity 57 is formed viahole conductors 58 for electrically connecting the top surface of the main body 51 and the ceiling surface of the cavity 57. Also, a sealing conductive pattern 59 for sealing a gap between the metal cover 54 and the top surface is formed on the top surface of the ceramic substrate 55. A wiring conductor 60 including an electrode pad for a controlling IC is formed on the ceiling surface of the cavity 57. Further, external terminal electrodes 61, 62 (63, 64) are formed on each of, for example, longer sides of the bottom surface of the rectangular hollow member 56. A plurality of recesses extending up to the bottom surface of the ceramic substrate 55 are formed in the opposite shorter sides of the main body 51, and terminal electrodes 65 to 68 are formed on the inner wall surfaces of these recesses. The terminal electrodes 65 to 68 are adapted to write temperature compensation data or other data in the IC chip 53.
The rectangular crystal oscillating element 52 is electrically coupled to the top surface of the main body 51 via electrode pads 69 and 70 using conductive adhesives 71, 72, and the metal cover 54 substantially in the form of a dish is integrally coupled using the sealing conductive pattern 59 in order to hermetically seal the crystal oscillating element 52. A controlling IC chip 53 is mounted in the cavity 57. This IC chip 53 is electrically connected to the electrode pad of the wiring conductor 60 via a bump or a bonding wire. In the cavity 57, resin 73 is filled and cured, so that the IC chip 53 is completely covered to have an improved resistance to humidity. In the aforementioned construction, the crystal oscillating element 52 mounted on the top surface of the main body 51 is connected with the IC chip 53 via the viahole conductors 58. The IC chip 53 and the IC control terminal electrodes 65 to 68 are connected via the wiring conductor 60 extending on the bottom surface of the ceramic substrate 55. Also, the IC chip 53 and the external terminal electrodes 61 to 64 are connected by a viahole conductor extending through the rectangular hollow member 56, or utilizing an inner wall surface of the rectangular hollow member 56.
The crystal oscillator thus constructed is manufactured by the following process.
First, the main body 51, the crystal oscillating element 52, the IC chip 53, etc. are prepared. Subsequently, the crystal oscillating element 52 is mounted on the electrode pads 69, 70 on the top surface of the main body 51, and is hermetically sealed by the metal cover 54. For example, a seam member is placed on the top surface of the main body 51 in advance and the metal cover 54 is placed to be seam-welded. Then, the IC chip 53 is mounted in the cavity 57 in a lower portion of the main body 51. Specifically, the IC chip 53 is bonded to the ceiling surface of the cavity 57 and is connected with the wiring conductor 60 including the IC electrode pad via an gold wire. Subsequently, the resin 73 is filled and cured in the cavity 57. Thereafter, specified temperature compensation data are written in the IC chip 53 using the IC control terminal electrodes 65 to 68 exposed to the outside of the main body 51.
However, in the aforementioned crystal oscillator, the IC chip 53 is required to be supplied with temperature compensation data to flatten a variation in the frequency of the mounted crystal oscillating element 52 in accordance with a temperature-frequency variation characteristic of the crystal oscillating element 52. The temperature compensation data is written in the IC chip 53 during the manufacturing process, and temperature compensation is actually performed after this oscillator is mounted on a printed circuit board. It will be seen that the oscillator does not function as a temperature compensating crystal oscillator at all if the temperature compensation data written in the IC chip 53 is erased or altered after the oscillator is mounted on the printed circuit board.
The IC control terminal electrodes 65 to 68 for writing the data in the IC chip 53 are formed in the opposite sides of the main body 51. More specifically, the terminal electrodes 65 to 68 extend to a bottom of the main body 51. When this oscillator is soldered to a printed circuit board after the temperature compensation data is written in the IC chip 53, accordingly, it is likely to occur that solder adheres to the IC control terminal electrodes 65 to 68 or an unexpected potential applies to the IC control terminal electrodes 65 to 68. Such an event causes the temperature compensation data in the IC chip 53 to be erased or altered. This is because the IC control electrode terminals 65 to 68 are provided in the opposite sides of the main body 51 and extend to the bottom of the main body 51. This problem can be seen to be avoided by providing the IC control terminal electrodes 65 to 68 sufficiently apart from the external terminal electrodes 61 to 64 to be soldered. This results in a larger size of the main body 51 or a largely reduced degree of freedom in a wiring of a printed circuit board to be connected with the oscillator.
In the conventional crystal oscillator, further, the electrode pads 69, 70 are formed on the top surface of the ceramic substrate 55 while the IC electrode pad is formed on the bottom surface of the is constructed as the ceramic substrate 55. The opposite ends of the viahole conductors 58 extending through the ceramic substrate 55 are exposed to the space for accommodating the crystal oscillating element 52 and to the cavity 57, respectively. In the case where there is no gap between the ceramic substrate 55 and the viahole conductors 58, i.e., the inner wall of the viahole or the through hole formed in the ceramic substrate 55 and the peripheral surface of the viahole conductors 58 are completely in close contact, there will be no problem. However, there are the likelihoods that: 1)Cracks occur due to a different thermal shrinkage coefficient between the ceramic substrate 55 and the viahole conductor 58; 2)The viahole conductor 58 is not completely in close contact with the ceramic substrate 55; 3) Even if complete airtight contact is attained before a crystal oscillating element 52 and an IC chip 53 are mounted, hermetic reliability is considerably reduced as result of a plurality of heat treatments, e.g., during mounting of the crystal oscillating element 52, during stabilization of the frequency of the crystal oscillating element 52, during mounting of the IC chip 53, or during the filling and curing of resin 73. Accordingly, there is a likelihood that the airtight space for accommodating the crystal oscillating element 52 communicates with the cavity 57 via undesirable gap or crack around the viahole conductor 58. As a result, the environment of the accommodation space enclosing the crystal oscillating element 52 changes, consequently changing the oscillation characteristic.
Further, according to the manufacturing process of the conventional crystal oscillator, the crystal oscillating element 52 and the IC chip 53 are separated or partitioned by the ceramic substrate 55 of the main body 51, and are separately mounted. However, even if an already mounted crystal oscillating element 52 has an oscillation characteristic which cannot be controlled by a temperature compensating circuit or has an oscillation defect, this improper or defective state cannot be detected until an IC chip 53 is mounted in the cavity 57 in a lower portion of the main body 51 and a measurement is made using the external terminal electrodes 61 to 64. Accordingly, if the crystal oscillating element 52 is improper or defective, the IC chip 53 mounted in the main body 51 also has to be thrown away. This is extremely disadvantageous in terms of cost.
In view of this problem, it may be appreciated to form, on the outer surface of the main body 51, monitor electrode pads for measuring the oscillation characteristic of the crystal oscillating element 52 after the crystal oscillating element 52 is mounted in the main body 51. However, the finally manufactured crystal oscillator has a problem of requiring additional treatment to prevent the monitor electrode pad from receiving unnecessary electric noise from outside after the oscillator is mounted on a printed circuit board.